This application is based upon and claims priority of Japanese Patent Application No. 2000-121436, filed on Apr. 21, 2000, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to the structures of DFB lasers capable of single-wavelength operation among semiconductor light-emitting devices principally used in optical communication systems. As the communication rate of optical communication systems is increased, semiconductor light-emitting devices having high wavelength stability are demanded. In particular, such semiconductor light-emitting devices are required to induce no mode hop even during modulation with high output power or even if external optical feedback exists.
2. Description of the Related Art
Conventionally, gain-coupled DFB lasers having periodic gain modulation in the direction of resonant cavity are proposed as a single-wavelength laser having high mode stability (such gain-coupled DFB lasers include complex-coupled DFB lasers characterized by having both of index coupling and gain coupling). Several methods are available for realizing gain modulation. Examples are a method of periodically modulating the thickness of active layers or guide layers, a method of forming periodic current-blocking layers adjacent to active layers, and a method of forming periodic light absorption layers adjacent to active layers. In particular, a structure (to be referred to as an MQW diffraction grating structure hereinafter) in which the number of multiple quantum well layers (MQW layers) as active layers is periodically changed has the advantages that a relatively large gain coupling coefficient can be ensured, the phases of gain coupling and index coupling match, and no extra absorption occurs.
FIGS. 1A to 1C show a conventional gain-coupled DFB laser using this MQW diffraction grating structure. FIG. 1A is a schematic sectional view of the main components of a semiconductor light-emitting device. FIG. 1B is a schematic sectional view of a region C in FIG. 1A. FIG. 1C shows a bandgap along a broken line D in FIG. 1B.
An MQW diffraction grating 102 includes periodically divided MQW layers (MQW-A) and flat MQW layers (MQW-B), and is formed between an n-InP substrate 101 and a p-InP cladding layer 103. The film characteristics and film thicknesses of well layers and barrier layers in MQW-A are the same as in MQW-B.
Referring to FIG. 1, of the total of six MQW layers, three upper layers are periodically divided to form MQW-A, and three lower layers are flattened to form MQW-B. In the MQW diffraction grating structure, a gain coupling coefficient, an index coupling coefficient, and the gain of the whole active layers can be controlled by changing the number of layers in each of MQW-A and MQW-B. Basically, the gain coupling coefficient and the index coupling coefficient increase when the number of layers in MQW-A is increased; the total gain of the active layers increases with no large changes in the coupling coefficients when the number of layers in MQW-B is increased.
A gain-coupled DFB laser having this MQW diffraction grating structure has the following several problems.
The first problem will be described below. In a gain-coupled DFB laser, the ratio of the gain coupling coefficient to the index coupling coefficient greatly contributes to mode stability. For example, in a uniform diffraction grating containing no phase shift in a resonant cavity, it is generally desirable that the ratio of the gain coupling coefficient to the index coupling coefficient be large. The values of these two coupling coefficients have influence on other various characteristics. For example, if these coupling coefficients are large, generally the lasing threshold current becomes small and the resistance to external optical feedback becomes high, but the slope efficiency lowers and the influence of the spatial hole-burning effect increases. Accordingly, it is desirable to control appropriately both the index coupling coefficient and the gain coupling coefficient in accordance with required device characteristics.
Unfortunately, in the conventional MQW diffraction grating structure shown in FIG. 1, the number of layers in MQW-A contributes to both the gain coupling coefficient and the index coupling coefficient. This makes it difficult to control these parameters independently. For example, if the number of layers in MQW-A is increased to increase the gain coupling coefficient, the index coupling coefficient increases at the same time. Consequently, the slope efficiency lowers, or the spatial hole-burning effect becomes strong.
The second problem of the conventional MQW diffraction grating structure shown in FIG. 1 is as follows. That is, when a differential gain (a change in gain with a change in carrier density) is increased to increase a modulation bandwidth, the dependence of the gain on the carrier density, i.e., the dependence of the gain coupling coefficient on the carrier density increases in MQW-A. This increases variations in the gain coupling coefficient during modulation.
The third problem of the conventional diffraction grating structure shown in FIG. 1 will be described below. When strain is introduced in MQW layers, this strain enters in different ways into layers of MQW-A and MQW-B (see FIG. 2), even if these layers have the same composition (i.e., the same lattice constant). This produces a difference between the gain spectra in MQW-A and MQW-B. For example, when compressive strain is introduced, the gain peak in MQW-A shifts to a shorter wavelength than in MQW-B; when tensile strain is introduced, the gain peak in MQW-A shifts to a longer wavelength than in MQW-B. When this is the case, it is difficult to set properly the gain peak position with respect to the lasing wavelength that is determined by the diffraction grating period. As an example, if the gain peak wavelength is too far from the lasing wavelength, the lasing threshold current increases, or the temperature characteristics deteriorate.
It is an object of the present invention to control independently the index coupling coefficient and the gain coupling coefficient and improve the mode stability without deteriorating the characteristics such as the lasing threshold current, slope efficiency, and the resistance to external optical feedback, in a semiconductor light-emitting device of a gain-coupled DFB laser using the MQW diffraction grating structure.
It is another object of the present invention to provide a semiconductor light-emitting device having the MQW diffraction grating structure, which has less dependence of the gain coupling coefficient on the carrier density while keeping the total differential gain of the active layers large, so as to achieve a large modulation bandwidth, and less wavelength variation during modulation.
It is still another object of the present invention to provide a semiconductor light-emitting device which, even if strain is introduced in multiple quantum well layers, can match the gain peak wavelength of first multiple quantum well layers (MQW-A) with that of second multiple quantum well layers (MQW-B), and which can appropriately set the gain peak position with respect to the lasing wavelength that is determined by the diffraction grating period, thereby preventing an increase in the lasing threshold current or deterioration of the temperature characteristics.
The present invention provides a semiconductor light-emitting device having a multiple quantum well structure (MQW structure) in which well layers are stacked via barrier layers, and which amplifies light by current injection, characterized in that the number of barrier layers and the number of well layers periodically change in the propagation direction of light in a partial region or the whole region of the multiple quantum well structure, and that the multiple quantum well structure comprises, in the region, first multiple quantum well layers (MQW-A) divided in the propagation direction of light by a period which is an integral multiple of the half wavelength of the propagating light in a medium, and second flat multiple quantum well layers (MQW-B), and that at least one of the film characteristics and film thickness of at least one of the barrier layers and the well layers in the first multiple quantum well layers is controlled to a desired condition different from that in the second multiple quantum well layers.
In this semiconductor light-emitting device, periodic gain modulation exists in the propagation direction of light and generates distributed feedback with respect to propagating light.
More specifically, in this semiconductor light-emitting device, periodic gain modulation and index modulation coexist in the propagation direction of light, and both the gain modulation and index modulation generate distributed feedback with respect to propagating light.
Furthermore, the difference of the film characteristics is achieved by controlling at least one of the material composition, doping with p- or n-type impurities, and introduction of compressive strain or tensile strain.
The relationships between the above modes of the present invention and the first to third problems described previously will be explained below.
To solve the first problem, the present invention can control the ratio of the gain coupling coefficient to the index coupling coefficient by making the structures of the well layers and/or the barrier layers in MQW-A different from those in MQW-B as described above. For example, the well layers in MQW-B are made thinner than the well layers in MQW-A. In this case, the quantum level in MQW-B shifts to higher energy than in MQW-A. This increases the ratio of the number of carriers injected into MQW-A to the number of carriers injected into MQW-B. Accordingly, the ratio of the gain of MQW-A to the total gain of the active layers increases, and this increases the gain coupling coefficient. On the other hand, the refractive index of MQW-A is lowered by this increase in the number of carriers, and this decreases the index coupling coefficient.
It leads to the same effect that the material compositions of the well layers in MQW-A and MQW-B are made different from each other to make the bandgap of the well layers in MQW-B larger than that of the well layers in MQW-A. The same effect can be expected by making the barrier height of the barrier layers in MQW-B higher than that of the barrier layers in MQW-A. It is also possible to decrease the gain coupling coefficient and increase the index coupling coefficient by switching the above structures of MQW-A and MQW-B.
To solve the second problem, in the present invention, the differential gain (the ratio of a change in the gain to a change in the carrier density) with respect to the lasing wavelength that is determined by the diffraction grating period is decreased in MQW-A and increased in MQW-B. In this manner, the dependence of the gain coupling coefficient on the carrier density is decreased while the differential gain of the whole active layers is kept large. For example, the gain peak wavelength of MQW-A is shifted to a shorter wavelength, and that of MQW-B is shifted to a longer wavelength, with respect to the lasing wavelength that is determined by the diffraction grating period. In this case, the differential gain is small in MQW-A because the lasing wavelength is longer than the gain peak wavelength. In contrast, the differential gain is large in MQW-B since the lasing wavelength is shorter than the gain peak wavelength.
Examples of means for setting the gain peak wavelengths in the above positions are: making the well layers in MQW-A thinner than the well layers in MQW-B; making the bandgap of the well layers in MQW-A larger than that of the well layers in MQW-B; and making the barrier height of the barrier layers in MQW-A higher than that of the barrier layers in MQW-B.
Even when the positions of the gain peaks in MQW-A and MQW-B with respect to the lasing wavelength do not satisfy the above relationship, it is possible to decrease the differential gain in MQW-A and increase the differential gain in MQW-B by appropriately designing the structures of the two MQW layers. For example, the differential gain in MQW-A can be decreased by introducing tensile strain in the well layers in MQW-A or doping MQW-A with n-type impurities. Also, the differential gain in MQW-B can be increased by introducing compressive strain in MQW-B or doping MQW-B with p-type impurities.
To solve the third problem, the present invention matches the gain peak wavelengths in MQW-A and MQW-B by making the structures of the well layers and/or the barrier layers in MQW-A different from those in MQW-B. For example, when compressive strain is to be introduced in both of MQW-A and MQW-B, the well layers in MQW-A are made thicker than the well layers in MQW-B so that the gain peak wavelengths in MQW-A and MQW-B match in accordance with the strain amount. When this is the case, the quantum level in MQW-A shifts to a longer wavelength because the film thickness is increased. This makes it possible to cancel the difference between the gain peak wavelengths in MQW-A and MQW-B produced by the difference between the ways the strain enters.
Also, even when compressive strain is to be similarly introduced, the same effect can be expected by setting, in accordance with the strain amount, the composition of the well layers in MQW-A such that the light emission wavelength when MQW-A is kept undivided is longer than that of MQW-B.
Additionally, even when compressive strain is to be similarly introduced, the same effect can be expected by making the barrier height of the barrier layers in MQW-A lower than that of the barrier layers in MQW-B in accordance with the strain amount. When tensile strain is to be introduced, on the other hand, the same effect can be expected by switching the structures of MQW-A and MQW-B in the above examples. Furthermore, when compressive strain or tensile strain is to be introduced, the same effect can be expected by setting the strain amount in MQW-B to be larger than the strain amount in MQW-A when MQW-A is kept undivided, in accordance with the strain amount in MQW-A.
When the present invention is applied to the first problem, it is possible to control independently the index coupling coefficient and the gain coupling coefficient in a semiconductor light-emitting device having an MQW diffraction grating structure. Accordingly, the mode stability can be improved without deteriorating the characteristics such as the lasing threshold current, slope efficiency, and the resistance to external optical feedback.
When the present invention is applied to the second problem, the dependence of the gain coupling coefficient on the carrier density can be decreased while the differential gain of the whole active layers is kept large. Accordingly, it is possible to implement a semiconductor light-emitting device with an MQW diffraction grating structure, which has a large modulation bandwidth and varies the gain coupling coefficient little during modulation.
When the present invention is applied to the third problem, the gain peak wavelengths of MQW-A and MQW-B can be matched even when strain is introduced in these MQW layers in a semiconductor light-emitting device with an MQW diffraction grating structure. As a consequence, the gain peak positions can be appropriately set with respect to the lasing wavelength determined by the diffraction grating period. This makes it possible to prevent an increase in the lasing threshold current and deterioration of the temperature characteristics.
In the semiconductor light-emitting device of the present invention, the index coupling coefficient and the gain coupling coefficient can be independently controlled. Therefore, the mode stability can be improved without deteriorating the characteristics such as the lasing threshold current, slope efficiency, and the resistance to external optical feedback.
Also, the dependence of the gain coupling coefficient on the carrier density can be decreased while the differential gain of the whole active layers is kept large. This makes it possible to increase the modulation bandwidth and decrease the wavelength variation during modulation.
Furthermore, even when strain is introduced in multiple quantum well layers, the gain peak wavelengths of first and second multiple quantum well layers can be matched. Consequently, the gain peak positions can be appropriately set with respect to the lasing wavelength determined by the diffraction grating period. Hence, it is possible to prevent an increase in the lasing threshold current and deterioration of the temperature characteristics.
As described above, the present invention can improve the various characteristics of a semiconductor light-emitting device and thereby implement a semiconductor light-emitting device with high wavelength stability, which does not induce any mode hop even during modulation with high output power or even when external optical feedback is present.